An example of a pitch shifting device that shifts the pitch of input sound by a predetermined pitch shift amount and outputs the pitch-shifted sound is described, for example, in Laid-open patent application HEI 8-171387. That reference describes a pitch shifter (i.e., a pitch shift device) that outputs more musically natural pitch-shifted sound. The pitch shifter stores, in advance, a table that specifies the amount of pitch shift for each pitch (C, C#, D, D#, . . . , A#, B) of an input sound, for adjusting the pitch after pitch shift so as to make it sound more musically natural. The stored pitch shift amount includes pitch shift amounts corresponding to a key and a scale at the time of performance, which can be set by the performer. The stored pitch shift amount also includes shift information indicative of a pitch (for example, +3rd, −3rd or the like). The pitch shifter changes the pitch of input sound using a pitch shift amount table, according to key information (a key and a scale) set by the performer and shift information.
FIG. 7 (a) is a schematic diagram showing an example of contents of a pitch shift amount table. The pitch shifter described in Laid-open patent application HEI 8-171387 uses a similar table. The pitch shift amount table shown in FIG. 7 (a) is a table that is referred to when the key information is set at C (C major) or Am (A minor), and the shift information is set at +third (+3rd). In the pitch shift amount table, pitches that are expressed by note names described in an upper row correspond to pitch shift amounts in the unit of a halftone described in a lower row, respectively. The upper row is referred to according to the pitch of an input sound, and the corresponding amount of pitch shift in the lower row is determined. The lower row of the table shown in FIG. 7 (a) describes numbers “3” and “4” as the amounts of pitch shift. Each of the numbers indicates “three halftones (minor 3rd)” or “four halftones (major 3rd),” respectively. In other words, in the exemplary pitch shift amount table, the pitch shift amount specified for the pitch of an input sound is either +three halftones (minor 3rd) or +four halftones (major 3rd). More specifically, when the pitch of an input sound is either “C#,” “D,” “D#,” “E,” G#,” “A,” “A#” or “B,” the pitch shift amount is specified to be +three halftones; and when the pitch of an input sound is “C,” “F,” “F#” or “G,” the pitch shift amount is specified to be +four halftones.
FIG. 7 (b) is a graph showing temporal changes in the pitch of each of the sounds when performed with an electronic guitar by an ordinary performance method (a fret changing method), when a harmony sound is to be added to the input sound using the pitch shift amount table shown in FIG. 7 (a). The horizontal axis of the graph shows time, and the vertical axis shows note names of the respective input sounds (pitches). According to the graph shown in FIG. 7 (b), when an input sound 101 of “G” is inputted at time t1, a harmony sound 102 of “B” that is four halftones above “G” based on the pitch shift amount table of FIG. 7 (a) is outputted together with the input sound 101. Thereafter, consider a case where the input sound 101 is changed, for example, from “G” to “A” at time t2 by an ordinary performance method (through changing the fret). In this case, as shown in FIG. 7 (b), as the input sound changes from “G” to “A” at time t2 as a boundary, the harmony sound 102 also changes from “B” to “C” at time t2 as a boundary.
When the input sound 101 is “G,” then “B” (which is four halftones above “G”) is a musically natural harmony sound; and when the input sound 101 is “A,” then “C” (which is three halftones above “A”) is a musically natural harmony sound. Therefore, by using the pitch shift amount table of FIG. 7 (a), natural harmony sounds can be automatically added to performance sounds of an electronic guitar.
However, when pitch shifting is performed using the pitch shift amount table, unnatural pitch changes can occur when finger bending is performed.
FIG. 7 (c) is a graph showing temporal changes in the pitch of each of the sounds when performed with an electronic guitar by a bending performance method, and when harmony sound is to be added to the input sound using the pitch shift amount table shown in FIG. 7 (a) according to a conventional method. The horizontal axis of the graph shows time, and the vertical axis shows note names of the respective input sounds. Consider, for example, a case where an input sound 101 of “G” is inputted at time t1 and, thereafter, finger bending is started at time t2a to change the pitch of the input sound 101 from “G” to “A” at time t2c. In this case, as pitch of the input sound 101 rises, the pitch of the harmony sound 102 rises from “B” at time t2a and thereafter, while keeping the pitch difference of four halftones with respect to the pitch of the input sound. However, at time t2b, when the pitch of the input sound 101 is detected as “G#,” the pitch of the harmony sound 102 is set at “B” that is three halftones above “G#,” based on the pitch shift amount table of FIG. 7 (a). In this case, the pitch of the harmony sound 102 abruptly drops, which results in a pitch change that sounds unnatural.